feat: Optionally send sessions when they're created

[timfish]

Closes #713

This PR adds an option to the MainProcessSession integration that forces sessions to be send immediately on creation. Apart from a few lines to add this option and call hub.captureSession() when enabled, the rest of the changes are to add all the additional session envelopes to the session e2e tests.

In the Electron main process:

import { init, Integrations } from '@sentry/electron';

init({
  dsn: '__DSN__',
  integrations: [new Integrations.MainProcessSession({ sendOnCreate: true })]
});